This invention relates to cast sputtering targets and, more particularly to an improved cast sputtering target for forming a magnetic alloy thin film to be used as a recording medium in a magneto-optic device.
Conventional magnetic alloy thin films containing heavy rare earth elements and transition metals are generally prepared by vacuum evaporation, sputtering and the like. The most common method is sputtering because it provides the best magnetic thin films. These films can be mass produced due to improvements in the magnetron sputtering process.
Formation of alloy thin films by sputtering is classified into the following three groups depending on the structure of the sputtering target:
1. The target can be a composite target;
2. A "multi-sputtering" target using a plurality of targets; or
3. An alloy target.
In the case of a composite target, the composition of the alloy thin film is controlled by varying the ratio of the areas of each of the elements on the target surface. Two types of composite targets are available. In the first, a metal chip is disposed on a target formed of a different metal and is known as a "chip-on-target". Such a chip-on-target is not suitable for forming magnetic thin films over an extended period of time because the surface area of the chips is reduced as the target wears. This reduction in surface area causes the composition of the resultant magnetic thin film to vary.
The second type of composite target is an "embedded-in-target" wherein one type of metal is embedded in a different base metal. An embedded-in-target is an improvement over a chip-on-target since magnetic thin films prepared using such a target have a uniform composition even when the target is sputtered for an extended period of time. This is due to the fact that the area of each material embedded in the base metal barely changes as a function of target wear. However, both the embedded-in-target and the chip-on-target have a shortcoming in that the deposition of each element on the target surface affects the final thin film composition in the film plane.
When a plurality of targets are used in accordance with a multi-sputtering method, each target is formed of a different metal. The targets are sputtered simultaneously to form an alloy film on a substrate. Radio frequency (RF) power, direct current (DC) power or some combination of RF and DC electric power is supplied to each of the targets to form the alloy film. An advantage of this method is that the alloy composition can be changed readily by controlling the power supplied to each target.
The multi-sputtering method also has some shortcomings. For example, a plurality of power sources must be provided in addition to the plurality of targets and it is necessary to take countermeasures to prevent radio frequency interference. Finally, it is difficult to prepare homogeneous alloy thin films over extended periods of time since the composition of the films is affected by the speed of deposition and the variation in film thickness distribution as the target wears. Deposition speed and film thickness distribution variation directly cause fine changes in composition and homogeneity of resulting alloy films.
Magnetic thin films resulting from sputtering of alloy targets have a homogeneous composition because such targets are macroscopically homogeneous in both the vertical and horizontal directions. Therefore, such alloy thin films have a homogeneous composition even when sputtering is repeated many times.
Alloy targets may also be classified as sintered or cast targets. Sintered targets having a large surface area can be formed irrespective of the type of material that is used. However, special techniques are necessary in order to reduce the content of impurities, such as oxygen and nitrogen to less than about 2000 ppm. Therefore, it is difficult to form high purity alloy thin films by sputtering sintered targets.
The oxygen content of a cast target can be lowered to less than about 500 ppm and high purity alloy thin films can be provided by sputtering cast targets. Cast targets are prepared by melting the desired metals in appropriate proportions and casting the melted material in an inert gas atmosphere or under vacuum. The impurity level is lower because there is no powder used in processing. In addition, impurities are removed as slag when the metals are melted.
The best method for industrially manufacturing alloy thin films is to sputter a cast target. However, alloy ingots or targets formed of heavy rare earth metals and transition metals tend to form intermetallic compounds and crack easily. It is difficult to form an alloy ingot in the shape of a target having a large surface area.
Accordingly, it is desirable to provide an improved sputtering target for use in preparing homogeneous alloy thin films that can be used in improved magneto-optic recording media.